1. Field of the Invention
This invention relates to steam turbine shells and the flange bolts or studs, nuts and washers therefor.
2. Description of the Prior Art
Steam and gas turbines often employ shells which are made of two approximately 180.degree. half circular, flanged segments which surround the turbine shaft and other components. The two segments are held together by a series of bolts or studs and nuts which, when tightened, compress the flanged portions of the shells to form a 360.degree. circle capable of containing hot, high pressure fluid as well as internal flow components including seals.
Turbines, after assembly, normally operate at high temperature and pressure for long periods of time, often for a duration of over 5 years between overhauls. During such period of operation there will occur events, such as start-up and shutdown, when thermal gradients create high stress in the shells well beyond the recognized steady state stress that exists during normal operation.
These stress conditions normally create a tendency for shells to gradually become non-round. When this occurs, there is an increase in stud and flange stress as these components exert great force to prevent the shells from becoming non-round.
When a turbine is shut down for disassembly the bolts or studs and nuts are untightened one-by-one to allow removal of the upper half shell to provide access to the internal components. As each bolt and nut is loosened or removed, the forces necessary to prevent the shell from springing to a non-round condition are increased on those bolts and nuts which have not yet been loosened.
These forces, when combined with the normal bolt force, cause a much greater friction load on the bolt threads, thus making loosening of the bolt or stud and nut very difficult. It is often necessary to burn off several expensive bolts or nuts to accomplish their removal, often involving more than a day's loss in the critical period of the turbine outage resulting in great cost.